Mix the yeast, ½ tsp of the sugar and 2 tbsp of lukewarm water in a small bowl. Stir to dissolve the yeast then leave it for about 10 minutes — by which time it should have started frothing.
Combine the flour, remaining sugar and salt in a large bowl. Make a well in the mixture and add 1 beaten egg, the yeast mixture and the oil. Make sure you scrape out all of the yeast mixture into the flour.
Measure out 250ml of lukewarm water — it should feel slightly warm if you dip your finger in, but not too hot, as that would kill the yeast.
Add about 200ml of water to the flour mixture and stir, initially with a spoon and then with one hand. Keep the other hand clean until you start kneading.
The dough should start to come together. If there are a lot of very dry bits, add a little water, very gradually so it doesn’t get too wet and sticky. The dough should be moist but not soggy and a little bit tacky. If you do add too much water you can add a little more flour to make it easier to handle, but try not to.
When the dough has come together, use it to wipe out the bowl and turn it out onto a lightly floured work surface. Knead for about 10 minutes — it may become more sticky, and if it does, try to avoid adding flour. Either use a little oil on your hands and the work surface or regularly scrape the latter with a dough scraper.
Once it’s smooth, silky and a bit springy, stretch the sides down and pull them together to form a ball. Lightly oil a large, clean bowl, then roll the dough ball lightly around the bowl so the surface is coated in the oil.
Cover your bowl with cling film or a clean tea towel, and leave in a warm place until the dough has doubled in size (1-2 hours). You’ll know it’s ready if, when you press it lightly with your finger, the indentation remains.
Line a baking sheet with baking paper. Turn the risen dough out onto a clean worksurface, weigh it and divide into 3 equal pieces. Roll these into long sausage shapes about 25cm long, tapering them slightly at each end. Lay them in front of you, parallel but with a couple of centimetres between each one. Pull them together at one end and braid them, tucking the ends in.
Place the loaf on the lined baking sheet and loosely cover with cling film or a tea towel. Leave it to rise in a draft-free place until puffy and billowy, about 30-45 minutes.
While it’s rising, heat the oven to 200C/180C/gas 6 and gently brush the remaining beaten egg all over the challah, getting into all the crevices. Sprinkle with seeds if using.
Bake for 25-30 minutes or until the bottom of the loaf is golden brown and it sounds hollow when you tap it. Check after about 15 minutes and if the top of the loaf has started to get too dark, cover it with foil. Remove from the oven and leave to cool on a wire rack before eating.